October 22, 2013

Herpes Virus Study Helps Confirm Humans Migrated ‘Out Of Africa’

By comparing an array of herpes simplex virus type 1 (HSV-1) strains, geneticists have found an alternate confirmation of the “out-of-Africa” hypothesis that describes the human Diaspora, according to a new report published in the journal PLoS ONE.

The researchers said they chose HSV-1 because it is easy to gather samples from volunteers, generally not deadly, and forms lifelong latent infections. Because the virus is spread by close contact, it conveniently tends to run in families.

In the study, scientists from the University of Wisconsin-Madison examined 31 strains of HSV-1 collected in North America, Europe, Africa and Asia. According to lead researcher Curtis Brandt, a professor of medical microbiology and ophthalmology at the university, "the result was fairly stunning.”

"The viral strains sort exactly as you would predict based on sequencing of human genomes," he explained. “We found that all of the African isolates cluster together, all the viruses from the Far East, Korea, Japan, China clustered together, all the viruses in Europe and America, with one exception, clustered together.”

"What we found follows exactly what the anthropologists have told us, and the molecular geneticists who have analyzed the human genome have told us, about where humans originated and how they spread across the planet,” Brandt added.

Geneticists are able to see how organisms are related by tracking singular changes in their genetic code. Once they know how quickly a certain genome changes, they can build a "family tree" that illustrates when certain variants had their last shared ancestor.

Previous studies have looked at HSV-1 through the lens of a single gene or a small cluster of genes. According to Brandt, this approach can be misleading.

"Scientists have come to realize that the relationships you get back from a single gene, or a small set of genes, are not very accurate,” he said.

Ironically, in trying to examine the HSV-1 genome as a whole, the Wisconsin team decided to break it into 26 pieces. Family trees were made for each piece and these trees incorporated into a single tree for the whole genome, Brandt said.

In addition to finding genetic groupings that paralleled predominant theories of human migration, the researchers also noted subtle intricacies of suggested migration patterns – particularly in the case of a strain found in Texas that looked Asian in origin, not European like all other American strains of the virus.

"How did we get an Asian-related virus in Texas?" asked study co-author Aaron Kolb.

The researchers said they suspected it came from a Native American whose ancestors had traversed the "land bridge" that stretched across the Bering Strait about 15,000 years ago.

"We found support for the land bridge hypothesis because the date of divergence from its most recent Asian ancestor was about 15,000 years ago,” Brandt said. "The dates match, so we postulate that this was an Amerindian virus."

In conclusion, Brandt said that the study "was clear support for the out-of-Africa hypothesis. Our results clearly support the anthropological data, and other genetic data, that explain how humans came from Africa into the Middle East and started to spread from there."

"There is a population bottleneck between Africa and the rest of the world; very few people were involved in the initial migration from Africa," he added. "When you look at the phylogenetic tree from the virus, it's exactly the same as what the anthropologists have told us."